[web] Fix --web-content-hash breaking cache when --source-maps is used #193601

When building a Flutter web app with both --web-content-hash and --source-maps (e.g., flutter build web --wasm --web-content-hash --source-maps or flutter build web --web-content-hash --source-maps), build/web/precache_manifest.json records "urlHashed": false and a mismatched "hash" for main.dart.<hash>.js:

{
  "url": "main.dart.216ed961.js",
  "hash": "fca793b5",
  "size": 6079674,
  "urlHashed": false
}

Root Cause

1. In _hashAndRenameWebOutput, flutter_tools computes contentHash (216ed961) from the initial main.dart.js bytes (when its trailing comment is still //# sourceMappingURL=main.dart.js.map), derives newBasename = 'main.dart.216ed961.js', and then rewrites the trailing sourceMappingURL directive inside main.dart.js in-place to //# sourceMappingURL=main.dart.216ed961.js.map before renaming the file on disk ( additionally stripping any trailing newline due to \s*$ matching \n before EOF).
2. Later, during WebReleaseBundle.build, updatePrecacheManifest hashes the final build/web/main.dart.216ed961.js bytes on disk—which now contain the rewritten sourceMappingURL comment—producing a different hash (fca793b5).
3. Because 'main.dart.216ed961.js'.contains('.fca793b5.') is false, updatePrecacheManifest sets "urlHashed": false and "hash": "fca793b5", breaking cache validation and precache manifest consumers that rely on urlHashed: true and filename/content hash parity.

Other Affected Files

• main.dart.<hash>.js (Dart2JSTarget and the JS fallback in --wasm builds): Affected when --source-maps is passed.
• main.dart.<hash>.mjs.map (Dart2WasmTarget): If a .mjs.map file is present alongside main.dart.mjs, Dart2WasmTarget previously derived jsSupportMapName = '$jsSupportName.map'.
• flutter_service_worker.js / worker.<hash>.js: Uses _hashAndRenameWebOutput with sourceMapFile if present.
• main.dart.<hash>.wasm: Not affected (_hashAndRenameWebOutput skips sourceMappingURL rewriting when isWasm is true because .wasm embeds main.dart.wasm.map in a binary custom section).

Proposed Fix (Hash Inversion)

In _hashAndRenameWebOutput (packages/flutter_tools/lib/src/build_system/targets/web.dart):

1. Hash sourceMapFile first (mapHash) and rename it via computeHashedBasename(oldMapBasename, mapHash, file.fileSystem) (producing main.dart.<mapHash>.js.map, main.dart.<mapHash>.mjs.map, worker.<mapHash>.js.map).
2. Rewrite //# sourceMappingURL=<newMapBasename> in file (using positive lookahead (?=\s*$) so trailing newlines are preserved).
3. Compute contentHash from the final bytes of file and rename file to main.dart.<contentHash>.js so its filename hash matches its on-disk bytes and updatePrecacheManifest records "urlHashed": true with the matching "hash".
4. Resolve mainJsMapName in Dart2JSTarget.buildFiles and jsSupportMapName in Dart2WasmTarget.buildFiles via _resolveHashedBasename.